1. Field of the Invention
The present invention relates to a respiration monitoring apparatus, and, more particularly, to a mask with a respiratory gas measurement site that obviates the need for the operator to separately assemble a respiratory gas measurement component to a mask and minimizes dead space and breathing circuit resistance.
2. Description of the Related Art
A variety of respiratory masks are known that have flexible seals and cover the nose, mouth, or both of a human patient. The mask seal, which is also commonly referred to as the cushion, creates a seal against the patient's face. Because of the sealing effect that is created, gases can be provided at a positive pressure within the mask for delivery to the airway of the patient.
The uses for such masks range from high altitude breathing, i.e., aviation applications, to mining and fire fighting applications, to various medical diagnostic and therapeutic applications. For example, such masks are used in the delivery of non-invasive positive pressure ventilation (NPPV) and the delivery of anesthesia. NPPV may be delivered as a continuous positive airway pressure (CPAP) or variable airway pressure, such as a bi-level pressure that varies with the patient's respiratory cycle or an auto-titrating pressure that varies with the monitored condition of the patient. Typical pressure support therapies are provided to treat a medical disorder, such as sleep apnea syndrome, in particular, obstructive sleep apnea (OSA), or congestive heart failure. NPPV also includes delivering life-supporting ventilation or ventilation that augments the patient's own respiratory effort.
Examples of conventional respiratory masks used in the medical field for providing a positive airway pressure to an airway of a patient are described in U.S. Pat. No. 5,243,971 to Sullivan et al., which teaches a bubble-type of patient interface in which the seal is attached to a shell and extends from the shell in a concave fashion. Other conventional masks are disclosed, for example, in U.S. Pat. No. 5,540,223 to Starr et al. and U.S. Pat. No. 6,467,483 to Kopacko et al.
A still further type of mask is described in U.S. Pat. No. 5,647,357 to Barnett et al. and U.S. Pat. No. 6,397,847 to Scarberry et al., which uses a gel material as the seal or cushion to maximize patient comfort and sealing properties. U.S. Pat. No. 4,971,051 to Toffolon teaches a mask in which the seal includes multiple flaps, again to optimize patient comfort and the sealing property.
The need to effectively titrate therapies, such as NPPV, based on clinical measures, such as the patient's arterial levels of carbon dioxide (PaCO2) and/or changes in those levels, are being increasingly recognized. Recent studies have shown that in patients with dead space to tidal volume ratios less than 0.65, PaCO2 and PetCO2 are highly correlated. Additionally, it has been shown that changes in end-tidal carbon dioxide levels correlate well with changes in PaCO2. It is changes in these levels that can be used to assess the effectiveness of changes in therapy.
The end-tidal carbon dioxide levels are determined from the patient's expiratory carbon dioxide gas. The patient's expiratory carbon dioxide gas is measured by gas analyzers that are typically categorized into two different types: (1) “non-diverting” or “mainstream” gas sampling systems; and (2) “diverting” or “sidestream” gas sampling systems. A mainstream gas sampling system includes a sample cell that is disposed along the main path of a breathing circuit through which a patient's respiratory gases flow. As a result, all of the patient's inspired and expired respiratory gases pass through the sample cell, which is also known as a “cuvette”. A gas sensing system, which includes the elements necessary for monitoring respiratory gases, is coupled to the sample cell to measure the constituents of gas passing through the sample cell. For infrared gas sensing, the gas sensing system includes a radiation source and detector. An example of such a conventional mainstream gas measurement system using infrared gas sensing is described in U.S. Pat. No. 4,914,720 to Knodle et al.
A sidestream gas sampling system transports a portion of sampled gases from the sampling site, which is typically a breathing circuit coupled to the patient's airway or directly at the patient's airway, through a sampling tube to the sample cell, where the constituents of the gas are measured by a gas sensing system. Gases are continuously aspirated from the sample site through the sampling tube and into the sample cell, which is located within a gas measurement instrument. Examples of conventional sidestream gas sampling systems are taught in U.S. Pat. No. 4,692,621 to Passaro et al.; U.S. Pat. No. 4,177,381 to McClatchie; U.S. Pat. No. 5,282,473 to Braig et al.; and U.S. Pat. No. 5,932,877 also issued to Braig et al.
The increasing sophistication of therapies, such as NPPV and the currently available gas monitoring technologies, make the combination of these two modalities attractive and potentially clinically beneficial for the management of patients suffering from respiratory failure. Their actual use in clinical practice is limited due to a number of factors that are methodological, technical, and educational in nature. Methodological problems relate to the selection of the proper patient interface, management of leaks around the mask seals, which complicates the interpretation of the measurement as well as adding to inconvenience and the added complexity of the gas measuring apparatus. Technical problems of combining gas measurement technologies and masks relate to the added deadspace of the additional breathing circuit components, such as the airway adapter portion and its associated rebreathing, the added pressure drop of the additional breathing circuit component and its affect on the therapy being delivered, and the need to provide for the possible use of either sidestream or mainstream gas sampling technologies. For example, the choice of a nasal, full face, or a mask covering nose, mouth, and eyes, in combination with other factors, determines the appropriate gas sampling method to use for CO2 measurement. The success of respiratory gas monitoring with therapies, such as NPPV, depends on understanding and controlling these factors.
Given these problems associated with monitoring respiratory gases from patients with masks, it is desirable to provide a mask with both mainstream and sidestream respiratory gas monitoring capabilities. Such a mask would be beneficial if it also minimized the added dead space and the pressure drop associated with the application of conventional gas monitoring approaches. Additionally, such a mask is beneficial if it also permits the effectiveness of therapies, such as NPPV, to be assessed and provides convenience and simplicity to the user, thereby encouraging greater use of such therapies.